Process of improving the properties of heat resistant alloys



PROCESS OF llVlPROVlN G THE PROPERTIES OF HEAT RESISTANT ALLOYS William H. Zibell, Paramus, and Henry J. Forrest, Morris Plains, N. i., assignors to Forbell Corporation, Teaneck, N. J a corporation of New Jersey No Drawing. Application April 13, 1955,

Serial No. 501,192

2 Claims. (Cl. 148-3) The present invention relates to a novel and improved process for improving the properties of heat resistant alloys, such as cobalt-base alloys, having good high-temperature properties with respect to stress-rupture, endurance and creep properties, enabling such alloys to be used at higher temperatures than usual, or under more severe conditions at the same temperatures.

The present invention has for its object the provision of a novel and improved process for the treatment of alloys resistant to high temperature, such as the cobalt base alloys, so as to improve their high-temperature characteristics especially with respect to ultimate tensile strength, elongation, machineability and uniformity of these properties. Alloys treated in accordance with the process of the present invention, are also characterized by a globular or spheroidal structure, as distinguished from the crystalline structure which usually is typical of such alloys, and are susceptible of being rolled or otherwise formed into the desired shapes. A further object is the provision of a process for the treatment of such alloys to improve their properties and better adapt them for use in such special applications as gasturbine blades, after-burner structures, and the like.

High-temperature alloys conventionally are usually characterized by a relatively low elongation, insufiicient ultimate tensile strength and a relative hard structure which prevents their being easily machined and substantially prevents their being rolled or otherwise being formed into the desired structural parts. Improvements in the properties of such alloys have usually been sought by varying the initial composition of the alloy, but according to the present invention, the alloy is remelted under special conditions and thereby develops its unusual and desirable properties.

Objects and advantages of the invention will be set forth in part hereinafter and in part will be obvious herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the steps, processes and compositions pointed out in the appended claims.

The invention consists in the novel steps, processes, compositions and improvements herein shown and described.

The process of the present invention is generally applicable to those high-temperature alloys which contain a large, and usually a major proportion of cobalt and a relatively large amount of chromium, together with other metallic constituents, usually of a refractory nature. Among the alloys which are especially adapted for improvement by the process of the present invention is the alloy, known commercially as Stellite 31 which has the general composition of chromium 23 to 28%, tungsten 6 to 9%, nickel 9 to 12%, iron less than 2%, carbon 0.45% to 0.60%, with the remainder essentially cobalt except for very minor amounts of silicon, manganese, aluminum and molybdenum. Such an alloy in actual commercial practice has a maximum ultimate tennited States Patent sile strength at ordinary temperatures of about 80,000 pounds p. s. i. with an 8% elongation, has a relatively coarse crystalline structure and a usual harness of 24 to 32 (Rockwell C). It also has the disadvantage that it cannot be formed by rolling and is machined only with difficulty and by the used or special tools, such as carbide-tipped tools. At higher temperatures, the alloy exhibits poor properties such that it cannot be used for the blades of gas turbines to operate at temperatures of 1600 F. and higher, and frequently the output of such a turbine is reduced solely on account of the temperature which can be withstood by the blades thereof.

According to the process of the present invention, the properties of a high temperature resistant alloy are markedly improved by melting the alloy in shot or other divided form in a furnace, such as an induction furnace, and continuing the heating well above the melting point of the alloy. Thus with Stellite 31, which is substantially completely melted at a temperature of from 2600 to 2700 F. the heating is continued until the temperature reaches from 3100" to 3600, preferably about 3300 F., or about 500 to 1000" above its melting point and preferably about 600 thereabove, the maximum temperature being determined by the volatization temperature of the constituent metals in the alloy and the time of heating.

fter the alloy has been heated to its melting point, and preferably after it has attained its higher temperature, the alloy is treated by adding to the crucible containing the molten alloy from 15 to 25% and preferably about 20% of a scavenging and cleansing agent which comprises very finely divided calcium carbonate, preferably in the form of an impalpable powder made from ground oyster shells which contain at least 97% calcium carbonate. The powder is vigorously stirred into the molten alloy, as the alloy boils or gives off gas, and this stirring is continued intermittently for a period of from about 4 to 10 minutes, preferably about 5 minutes, thereby preparing the alloyfor pouring.

While the alloy is still heated to its high temperature and before the alloy has cooled to less than 5000 or preferably before it has cooled to less than 3200 F. the alloy is poured into molds which have been preheated to a temperature of at least 1500 and are preferably at substantially a temperature of 1800 F. These molds are preferably formed of some non-reactive refractory material, such as zirconia, and the cast alloy is allowed to remain therein until it has cooled to a relatively low temperature, such as 800", which may take as long as an hour.

The resulting re-cast alloy has substantially the same composition, with respect to the principal elements, as were originally present in the alloy, except that a marked reduction is noted in the silicon and carbon content of the alloy, and usually there has been a relatively great reduction in the quantity of absorbed or combined gases, absorption of additional gas having been prevented during the cooling process by the relatively thick layer of slag which remained on the upper surface of the molten alloy as it cooled. Additionally, the alloy has become almost completely spheroidized in its structure.

With Stellite 31, the following improvement in properties are developed by the process of the present invention: the tensile strength is increased from about 80,000 p. s. i. to well over 100,000 p. s. i. and often to as high as 106,000 or 107,000 p. s. i., and at such tensile strengths an elongation of 16 to 18% is found, as distinguished from the usual 8% found in the untreated alloy. The alloy treated in accordance with the present invention, has a markedly reduced hardness, usually about to 96 (Rockwell B) as distinguished from the normally occurring hardness of 24 to 32 (Rockwell C). As a result,

the treated alloy is readily'machineable by ordinary tools and may be rolled or otherwise formed into-the desired shapes. The treated alloy also exhibits an almost completely globular microscopic structure, as distinguished from the relatively coarse crystalline structure with numerous inclusions which normally characterize the alloy.

Typical alloys produced in accordance with the process of the present invention had ultimate analysis of:

Percent Percent Carbon 0. 34 0. 35 Molybdenum. 0. 10 0. 12 Chromiun1 25.00 25.00 Manganese 0. 37 0. 35 Silicn 0.22 (l. 20 Tungsten 7. 56 7. 71 Ir0n 1.16 1.00 Cobalt 53. 90 53. 70 Nickel 11.70 11. 00

the specific steps, processes and compositions shown and described but departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. A process for improving the physical properties of a high-temperature cobalt-base alloy which comprises remelting the alloy at a temperature from 3100 to 3600 F. stirring into the molten alloy from 15 to 25% of powdered calcium carbonate, pouring the alloy into a mold preheated to at least 1500 F. and slowly cooling the cast alloy.

2. A process for improving the physical properties of a high-temperature cobalt-base alloy which comprises remelting the alloy at a temperature of approximately 3300 F, stirring into the molten alloy from 15 to 25 of powdered calcium carbonate, pouring the alloy into a mold preheated to approximately 1800 F. and slowly cooling the cast alloy.

References Cited in the file of this patent UNITED STATES PATENTS 1,614,862 Beatty Jan. 18, 1927 1,731,346 Meehan Oct. 15, 1929 FOREIGN PATENTS 634,870 Great Britain Mar. 29, 1950 

1. A PROCESS FOR IMPROVING THE PHYSICAL PROPERTIES OF A HIGH-TEMPERATURE COBALT-BASE ALLOY WHICH COMPRISES REMELTING THE ALLOY AT A TEMPERATURE FROM 3100 TO 3600* F. STIRRING INTO THE MOLTEN ALLOY FROM 15 TO 25% OF POWDERED CALCIUM CARBONATE, POURING THE ALLOY INTO A MOLD PREHEATED TO AT LEAST 1500* F. AND SLOWLY COOLING THE CAST ALLOY. 